LeetCode-in-Java

1706. Where Will the Ball Fall

Medium

You have a 2-D grid of size m x n representing a box, and you have n balls. The box is open on the top and bottom sides.

Each cell in the box has a diagonal board spanning two corners of the cell that can redirect a ball to the right or to the left.

• A board that redirects the ball to the right spans the top-left corner to the bottom-right corner and is represented in the grid as 1.
• A board that redirects the ball to the left spans the top-right corner to the bottom-left corner and is represented in the grid as -1.

We drop one ball at the top of each column of the box. Each ball can get stuck in the box or fall out of the bottom. A ball gets stuck if it hits a “V” shaped pattern between two boards or if a board redirects the ball into either wall of the box.

Return an array answer of size n where answer[i] is the column that the ball falls out of at the bottom after dropping the ball from the ith column at the top, or -1 _if the ball gets stuck in the box._

Example 1:

Input: grid = [[1,1,1,-1,-1],[1,1,1,-1,-1],[-1,-1,-1,1,1],[1,1,1,1,-1],[-1,-1,-1,-1,-1]]

Output: [1,-1,-1,-1,-1]

Explanation: This example is shown in the photo.

Ball b0 is dropped at column 0 and falls out of the box at column 1.

Ball b1 is dropped at column 1 and will get stuck in the box between column 2 and 3 and row 1.

Ball b2 is dropped at column 2 and will get stuck on the box between column 2 and 3 and row 0.

Ball b3 is dropped at column 3 and will get stuck on the box between column 2 and 3 and row 0.

Ball b4 is dropped at column 4 and will get stuck on the box between column 2 and 3 and row 1.

Example 2:

Input: grid = [[-1]]

Output: [-1]

Explanation: The ball gets stuck against the left wall.

Example 3:

Input: grid = [[1,1,1,1,1,1],[-1,-1,-1,-1,-1,-1],[1,1,1,1,1,1],[-1,-1,-1,-1,-1,-1]]

Output: [0,1,2,3,4,-1]

Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 100
• grid[i][j] is 1 or -1.

Solution

@SuppressWarnings("java:S135")
public class Solution {
    public int[] findBall(int[][] grid) {
        int m = grid.length;
        int n = grid[0].length;
        int[] res = new int[n];
        for (int j = 0; j < n; j++) {
            int currentJ = j;
            int currentI = 0;
            while (currentJ < n && currentI < m) {
                if (grid[currentI][currentJ] == 1) {
                    currentJ++;
                    if (currentJ < n && grid[currentI][currentJ] == 1) {
                        currentI++;
                    } else {
                        break;
                    }
                } else {
                    currentJ--;
                    if (currentJ >= 0 && grid[currentI][currentJ] == -1) {
                        currentI++;
                    } else {
                        break;
                    }
                }
            }
            if (currentI == m) {
                res[j] = currentJ;
            } else {
                res[j] = -1;
            }
        }
        return res;
    }
}

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